The present invention relates to an improved branched polycarbonate and method for making said polycarbonate by incorporating a branching agent in the vapor phase prior to solid state polymerization (SSP). More particularly, the present invention provides a method for the preparation of branched polycarbonate by solid state polymerization wherein a mixture of branching agent and partially crystalline precursor polycarbonate is prepared by exposure of an unbranched, amorphous precursor polycarbonate to a vapor phase mixture comprising a solvent and branching agent such as 1,1,1 -tris(4-hydroxyphenyl)ethane (THPE). The treated precursor is then converted into a branched polycarbonate by SSP.
Polycarbonate resins are widely utilized, including in blow molding applications. For such applications, polycarbonates must possess high melt strengths, high shear sensitivities and high complex viscosity ratios. A certain threshold degree of internal branching is necessary for polycarbonates to achieve such characteristics. Historically, two basic methods have been used for the preparation of branched polycarbonates: interfacial polycondensation and melt phase carbonate interchange. Both methods typically employ polyhydric phenols such as THPE as branching agents.
Under interfacial polycondensation conditions, a dihydroxyaromatic compound together with a branching agent is contacted with phosgene in an aqueous-organic solution, mixed with an acid acceptor and an amine catalyst. Alternatively, the method involves the interfacial preparation of oligomeric chloroformates, which are then converted to high molecular weight polycarbonate by partial chloroformate group hydrolysis and polycondensation.
Branching agents, such as 1,1,1-tris-(hydroxyphenyl)ethane (THPE); and 1,3,5-tris-(4-hydroxyphenyl)benzene are polyhydric phenols having at least three hydroxy groups per molecule, have been used to prepare high melt strength blow moldable polycarbonate resins by interfacial polycondensation. Numerous other branching agents, including cyanuric chloride; and 3,3-bis-(4-hydroxyphenyl)oxyindoles, have also been used, as well as 1,2,3-trihydroxybenzene; 1,3,5-trihydroxybenzene; 1,3,5-tris(2-hydroxyethyl)cyanuric acid; 4,6-dimethyl-2,4,6-tris(4-hdroxyphenyl)heptane; 2,3,4-trihydroxyacetophenone; 2,3,4-trihydroxybenzophenone and 2,4,4xe2x80x3-trihydroxybenzophenone.
There are significant disadvantages inherent in the interfacial polycondensation process. First, toxic and hazardous phosgene is utilized as the source of carbonate units in these reactions. Also, the interfacial polycondensation process employs a chlorinated hydrocarbon, such as methylene chloride, as the organic solvent which requires substantial and costly environmental management to prevent unintended solvent emissions. Furthermore, the product polycarbonate contains residual sodium and chloride ions which adversely affect the hydrolytic stability of the product.
Alternatively, branched polycarbonates can be prepared from melt phase carbonate interchange reactions. Generally in a melt phase process, a bisphenol and a diaryl carbonate are contacted in the melt at a temperature in a range between about 270 and about 350xc2x0 C. in the presence of a suitable melt polymerization catalyst. An oligomeric polycarbonate is thereby produced, usually with an average molecular weight between 2,000 and 10,000 as determined by gel permeation chromatography, which can be relative to polycarbonate or polystyrene. The produced oligomer is then converted to a high molecular weight polycarbonate increasing the polymerization temperature and reducing the pressure of the reaction vessel. Branching agents used in melt phase processes include THPE, triphenyl trimellitate, triglycidyl isocyanurate, and 3,3-bis-(4-hydroxyphenyl)oxyindoles.
It is clear that melt phase processes suffer from a number of disadvantages as well. At high conversions ( greater than 98%), viscosity of the melt increases significantly. This is a major disadvantage because handling of high viscosity melt polymerization mixtures at high temperature is extremely difficult. Increased viscosity of the polymerization mixture leads to poor mixing and the generation of hot spots which can lead to the loss of product quality. Also, this process requires custom equipment such as a Helicone mixer operating at temperatures in the range of 270 300xc2x0 C. and capable of operation at subambient pressures.
Recent developments have resulted in the use of solid state polymerization as an alternative vehicle for preparing high-molecular weight polycarbonates. SSP reactions occur at substantially lower temperatures, specifically within the range of 180 230xc2x0 C. Solid state polymerization is amenable to the use of relatively low molecular weight polycarbonate oligomers having very low melt viscosities, produced via ester interchange, which are subsequently converted to high molecular weight polycarbonate in the solid state. Hence, the SSP process does not require handling of viscous, molten polycarbonate melt at high temperatures during the polymerization step. Moreover SSP permits the avoidance the large scale use of both phosgene gas and halogenated solvents such as methylene chloride. Finally, the SSP process step itself does not require specialized custom equipment.
Solid state polycondensation processes entail subjecting a suitable oligomer to programmed heating at a temperature above its glass transition temperature and below its sticking temperature while removing the volatile by-product. The polycondensation reaction can proceed strictly in the solid state under these conditions.
The typical SSP process to form a branched polycarbonate is twofold. First, a low melt viscosity linear oligomer is synthesized by the melt phase reaction of a bisphenol with a diaryl carbonate. Generally, a mixture of a dihydroxydiaryl compound and a diaryl carbonate is heated at about 150xc2x0 C. to about 325xc2x0 C. for a period of from about 4 to about 10 hours in the presence of a transesterification catalyst. This affords a polycarbonate oligomer having a weight average molecular weight in a range between about 2,000 and about 20,000 Daltons, said polycarbonate oligomer having both hydroxyl and carbonate end groups. Second, crystallization of the linear polycarbonate oligomer is effected by (a) dissolving the oligomer, a branching agent and a suitable catalyst in a solvent such as chloroform and then evaporating the solvent; (b) suspending the oligomer in diluent containing a branching agent and refluxing it for 0 to 10 hrs with a suitable catalyst and evaporating the diluent; or (c) heating the mixture of linear oligomer and branching agents at a temperature which is higher than the glass transition temperature of the polycarbonate and below its melting point in the presence of a catalyst. Each of crystallization methods (a), (b) and (c) to affords a partially crystalline mixture of linear polycarbonate oligomer, branching agent and catalyst which is suitable for use in solid state polymerization.
Typically, aliphatic aromatic hydrocarbons, ethers, esters, ketones, and halogenated aliphatic and aromatic hydrocarbons are preferred as solvents and diluants. The resulting oligomer should have a crystallinity range of between 5% and 55% as measured by a differential scanning calorimeter. At crystallinity levels above 55% the polymerization rate under solid state polymerization conditions is too low to be practical. Crystallinity level below 5% will likewise result in low or negligible solid state polymerization rates due to fusion of the precursor polycarbonate.
The crystallized oligomer, the branching agent and a suitable catalyst produced by methods (a), (b) or (c) above are heated under SSP reactions. The reaction temperature and time may vary according to the type (chemical structure, molecular weight, etc.) of partially crystallized oligomer as well as the physical form of the partially crystallized oligomer (pellets, powder, flake and the like). However, the polymerization temperature should be at least above the glass transition temperature of the polycarbonate and below the melting or sticking point of the oligomer. At this temperature the oligomer should not fuse during the solid state polycondensation. Since both the glass transition temperature and the melting point of the crystallized oligomer increases during the course of polycondensation, it is therefore desirable to increase the polycondensation temperature gradually. Generally the temperature should be 10-50xc2x0 C. below the melting point of the oligomer and it should be in the range of 150-250xc2x0 C. and more preferably between 180 and 220xc2x0 C.
In order to allow the SSP reaction to progress, the by-product, such as phenol or diphenol carbonate should be removed from the reaction system during the solid state polycondensation process. An inert gas is passed through the system to aid in the removal of the by-product. The inert gases which are generally used are N2, He, Ar and the like, and the flow rate of the inert gases is varied depending upon the type of reactor and the particle size of the oligomer. In a laboratory setting, flow rates of between 0.1 and 5 liters per minute are typical. The rate of polycondensation may be strongly dependent on the identity and the flow rate of the carrier gas.
After solid state polymerization has been completed, the amount of branching agent incorporated in the polycarbonate can be determined by complete hydrolysis of the polycarbonate followed HPLC analysis of the hydrolysis products using an internal standard. Any unreacted branching agent may be removed prior to hydrolysis by dissolution of the branched polycarbonate in a solvent such as chloroform and reprecipitation by addition to methanol or ethyl acetate.
The present invention addresses the foregoing problems, and provides further surprising advantages and properties. These and further objects of the invention will be more readily appreciated when considering the following disclosure and appended claims.
In one aspect, the present invention relates to a method for preparing a branched polycarbonate prepared via SSP. The method first involves the exposure of a solid, amorphous polycarbonate precursor to a mixture comprising solvent and branching agent vapors which results in the production of a partially crystalline polycarbonate precursor containing said branching agent. This partially crystalline precursor containing the branching agent is then subjected to SSP to form a branched polycarbonate.
The present invention further relates to a method for preparing branched polycarbonate wherein THPE is used as a branching agent said THPE being incorporated into a mixture comprising the partially crystalline precursor polycarbonate and the THPE branching agent via exposure of a linear, amorphous polycarbonate to a mixture comprising solvent vapors and THPE vapors.